The present disclosure relates generally to drilling systems and more particularly to downhole drilling tools and the manufacture thereof.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Wells are generally drilled into the ground or ocean bed to recover natural deposits of oil and gas, as well as other desirable materials that are trapped in geological formations in the Earth's crust. A well is typically drilled using a drill bit attached to the lower end of a “drill string.” Drilling fluid, or “mud,” is typically pumped down through the drill string to the drill bit. The drilling fluid lubricates and cools the drill bit, and it carries drill cuttings back to the surface in an annulus between the drill string and the borehole wall.
For successful oil and gas exploration, it is necessary to have information about the subsurface formations that are penetrated by a borehole. For example, one aspect of standard formation evaluation relates to measurements of the formation pressure, formation permeability and the recovery of formation fluid samples. These measurements are essential to predicting the economic value, the production capacity, and production lifetime of a subsurface formation.
One technique for measuring formation properties includes lowering a “wireline” tool into the well to measure formation properties. A wireline tool is a measurement tool that is suspended from a multi-wire cable as it is lowered into a well so that is can measure formation properties at desired depths. A typical wireline tool may include a probe that may be pressed against the borehole wall to establish fluid communication with the formation. This type of wireline tool is often called a “formation tester.” Using the probe, a formation tester measures the pressure of the formation fluids and generates a pressure pulse, which is used to determine the formation permeability. The formation tester tool also typically withdraws a sample of the formation fluid for analysis within the tool and/or for later analysis.
In order to use any wireline tool, whether the tool is a resistivity, porosity, or formation testing tool, the drill string must be removed from the well so that the tool can be lowered into the well. This is called a “trip” downhole. Further, wireline tools must be lowered to the zone of interest, generally at or near the bottom of the hole. A combination of removing the drill string and lowering the wireline tools downhole are time-consuming measures and can take many hours and even days, depending upon the depth of the borehole. Because of the expense and rig time required to “trip” the drill pipe and lower the wireline tools down the borehole, wireline tools are generally used only when the information is greatly desired. A drill string may be tripped for other reasons, such as changing the drill bit.
As an improvement to wireline technology, techniques for measuring formation properties using tools and devices that are positioned near the drill bit in a drilling system have been developed. Thus, formation measurements are made during the drilling process, and the terminology generally used in the art is “MWD” (measurement-while-drilling) and “LWD”(logging-while-drilling). MWD typically refers to measuring the drill bit trajectory, as well as borehole temperature and pressure, while LWD typically refers to measuring formation parameters or properties, such as resistivity, porosity, permeability, and sonic velocity, among others. Real-time data, such as the formation pressure, allows the drilling company to make decisions about drilling mud weight and composition, as well as decisions about drilling rate and weight-on-bit, during the drilling process.
Recently, the equivalent of wireline formation testing tools have been introduced on the drill string: tools capable of measuring the pressure and permeability of formations and capable of extracting large volumes of formation fluids and capturing representative samples. Unlike wireline formation testers, these drilling formation testers are confined, due to the harsh (pressure, temperature, shock and vibration, etc.) environment under which drilling takes place, to operate within collars. Such collars constitute a part of a drilling bottom-hole assembly. Collars, amongst other functions, serve to house and protect the drilling formation tester while allowing the passage of mud past the drilling formation tester on its way to the drill bit at the bottom of the drilling assembly. The commonly-used approach of housing the drilling formation tester within a single collar can place restrictions on the utility of these tools: For example, with a fixed collar configuration (e.g., having a fixed diameter and length), the range of borehole sizes within which the tool may be successfully operated is generally narrow. In addition, the choice of probe types which may be used to address wide ranging pressure testing and sampling conditions is limited unless multiple variants of these relatively expensive collars are kept in inventory. Further, not all applications of these tools utilize identical tool configurations.
A sampling formation tester having a fixed, inflexible configuration can result in less than optimal performance. Further, at significant depths, substantial hydrostatic pressure and high temperatures are experienced, thereby further complicating matters. Still further, formation testing tools are operated under a wide variety of conditions and parameters that are related to both the formation and the drilling conditions. Therefore, there is a need for improved downhole formation evaluation tools and improved techniques for operating and controlling such tools so that such downhole formation evaluation tools are more reliable, efficient, and adaptable to both formation and mud circulation conditions.